In a Fischer-Tropsch reactor (in the following also abbreviated FT reactor), hydrogen and carbon monoxide are reacted in the presence of a transition metal catalyst, such as cobalt or iron, to form a composition containing a broad range of hydrocarbons.
Typically, the effluent of a Fischer-Tropsch reactor will comprise hydrocarbons which are liquid or solid or semi-solid at ambient temperature and pressure (the latter also called “waxes”). Such hydrocarbons can be processed to fuel class, for example diesel oil, hydrocarbon compositions, or alternatively the hydrocarbons can be converted to intermediate compositions from which fuel class hydrocarbons can be produced. In addition the reactor effluent will comprise hydrocarbons which are gaseous at ambient temperature and pressure. The latter kind of hydrocarbons is represented by methane and hydrocarbons of typically 2 to 4 carbon atoms.
A number of carbonaceous sources have been used as raw-materials for producing hydrogen and carbon monoxide containing synthesis gas (hereinafter referred as to syngas) which can be fed into the FT process. Originally, coal was used as the primary raw-material, but lately also natural gas has been taken into use in commercial processes. Even more recently various processes have been developed in which biological materials, such as plant oils, plant waxes and other plant products and plant parts or even oils and waxes of animal origin, are gasified and processed to produce a suitable feed. In a further alternative approach, viz. in the BTL process (biomass to liquid process), a biomass comprising whole plants is used as a raw-material. The BTL process allows for the utilization of forestry residues.
A BTL process can include the steps of biomass feed pre-treatment, biomass gasification, raw syngas cooling and filtering, raw gas purification, shift reaction for balancing H2/CO ratio, FT-process and FT product refining. For gasification of the biomass oxygen steam can be used (e.g. by blowing it into the gasification zone) for minimizing inerts in syngas.
In gasification, can be preferred to use steam or oxygen or combinations thereof blown into the gasification zone for fuel production by the FT process. A typical temperature range is about 700 to 950° C. At these conditions, biomass, such as lignocellulosic materials, will produce a gas containing carbon monoxide, carbon dioxide, hydrogen and water gas. It further contains some hydrocarbons and impurities, such as sulphur and trace metals.
Whereas a gasifier operated at the above conditions can produce a gas having a molar ratio of hydrogen to carbon monoxide of about 0.5 to 1.4 depending on feedstock, the Fischer-Tropsch reactor can require a higher molar ratio of about 2:1. Therefore, it can be desirable to increase said ratio in the gas produced in such a gasifier.
For this purpose, it is possible to carry out gasification at a higher temperature of, for example, 950° C. or more. Higher gasification temperatures decreases crease tarry side-products in raw syngas but may cause sintering problems for gasifier.
Another option to reduce tarry side products in raw syngas is to feed the gas into a catalytic reformer (see patent application FI20105201) wherein the gas is subjected to further thermal reactions which give a product mix containing an increased portion of carbon monoxide. The gaseous effluent of the reformer has to be freed from carbon dioxide, water and catalyst poisons before it can be used as a syngas for a FT reaction. Furthermore, the hydrogen-to-carbon monoxide ratio may need to be even further increased.
It is possible to achieve the latter goal by subjecting the gas to a water gas shift (WGS) reaction. In the shift reaction hydrogen is produced by reacting carbon monoxide with water to produce carbon dioxide and hydrogen.
The art is impaired by considerable problems. Thus, catalysts used in shift reactions are notoriously sensitive to impurities of the kind produced by gasification of biomass. Examples of such impurities include hydrosulphide (H2S), ammonia (NH3), hydrochloride (HCl), hydrogen cyanide (HCN) and particles which act as catalyst poisons and/or inhibitors for reactions. Small solid coke- and crystallized phosphor-containing particles are typically also present in the raw syngas.
Generally, impurities and particles will accumulate in any shift and hydrolysis catalyst bed reactors and cause problems in terms of corrosion and plugging and deactivation of the catalysts. The concentration of particles and impurities and other catalyst poisons in the inlet to the shift reactor therefore have to be strongly reduced, normally by extensive purification. The shift reaction will also have to be carried out at rather high temperature to limit catalyst poisoning.
As shift reactor catalyst there exists two different types of catalyst in the market, one for very low sulphur level gas shift, typically for less than one ppm hydrogen sulphide level for feed gas and an other for high sulphur level feeds, typically for 200 ppm hydrogen sulphide level in feed. Many biobased feedstock as wood residuals have sulphur level that yields to 50-200 ppm hydrogen sulphide level in syngas after gasification and reforming steps. These sulphur level syngas can be either purified to meet low sulphur level shift catalyst requirements or extra hydrogen sulphide can be added to shift gas feed to meet high sulphur level shift catalyst requirements.
For the above reasons, it is difficult economically to carry out an efficient shift reaction using raw syngas on an industrial scale.